The relative atomic mass of an element is the average mass of its
isotopes compared with an atom of carbon-12.

Isotopes occur when atoms have different numbers of neutrons but the
same number of protons.

A mole is a specific amount of atoms. One mole equals 6.02 x 10²³
atoms. This number is known as Avogadro's number.

A mole of a substance is obtained by weighing out the RAM or formula mass
in grams. This is because, for example, 12 grams of carbon contains 6.02
x 10²³ atoms. 18 grams of water would contain 6.02 x 10²³
molecules of water, i.e. 1 mole.

The formula of a compound tells you how many atoms of each element
combine.

The empirical formula is the simplest ratio of atoms in a compound.